The Cooking Hypothesis is quite popular, especially among journalists and vegans but also in the general nutrition-savvy public. It usually serves to support a high plant diet early in human evolution, which is the original claim of the hypothesis.
As the hypothesis comes up in many Twitter discussions, I thought it might be useful to publish here a section of my Ph.D. thesis that deals with the Cooking Hypothesis. Richard Wrangham is a first-rate scientist for whom I have much respect, so I do not want this post to be interpreted as non-appreciative of his work. Science, however, is a method that relies on constant reassessments of hypotheses. There are always more hypotheses regarding a certain phenomenon than true statements so by definition most hypotheses are wrong. One of our tasks as scientist is to try to assess the probabilities of the various hypotheses being right or wrong. And one last statement regarding bias. I accept the Paleo mismatch theory. Not only do I accept it but I also feel I must present it to as many people as I can. This position may create a bias in my interpretation of evidence. In the same manner, it is relevant to note that Richard Wrangham stated in the past that he is a vegetarian. I do not know if this fact creates a bias in his interpretation of the evidence.
You may also like to read another debunking attempt by Cornelio et al. (Cornélio et al., 2016), that, unlike me, also did some lab work. And now to a section of my Ph.D. thesis, that deals with the cooking hypothesis:
Teeth, in line with the rest of the masticatory system, should closely reflect the physical form of the diet, as the masticatory action is repeated thousands of times each day, and is thus subject to continuous pressure to adjust to efficiently process the diet (Lucas et al., 2009).
One of the main derived features of Homo is the reduction in the relative size of all the components of the masticatory apparatus (Aiello and Wheeler, 1995). That reduction has been associated with a substantial decrease in feeding time (approx. 5% of daily activity in humans compared to 48% of daily activity in chimpanzee) starting, as argued, with H. erectus 1.9 Mya (Organ et al., 2011).
The masticatory system size together with feeding time reductions have been attributed by researchers to the increased proportion of meat in the diet (e.g., Aiello and Wheeler, 1995; Zink and Lieberman, 2016), high proportion of fat in the diet (Ben-Dor et al., 2011) or to the introduction of cooking, early in Homo evolution (Wrangham et al., 1999; Wrangham and Conklin-Brittain, 2003; Wrangham and Carmody, 2010; Wrangham, 2016; Wrangham, 2017). While it is agreed that the reduction in the size of the masticatory apparatus and in feeding time reflects a diet that became more energy-dense in relation to volume, its meat-plant ratio cannot be deduced if one accepts the cooking hypothesis, which proposes to bring fibrous, starchy plant tubers into the energy-dense food category already 1.8 Mya, when the relatively small toothed H. erectus appeared (McHenry, 2009).
In their initial presentation of the hypothesis, Wrangham et al. (1999) claimed that wild meat is a low-fat food, which may have low nutritional quality during lean periods, based on research by Speth and Spielmann (1983) and Speth (1989). After excluding several plant food alternatives as too seasonal or too fibrous, Wrangham et al. focused on underground storage organs (USOs) (Wrangham et al., 1999). They state, in agreement with Stahl et al. (1984), that the starch of uncooked USOs (potatoes in their example) is resistant to amylase, the human enzyme that turns starch into usable energy. They proposed that the cooking of USO must have been practiced by Homo erectus 1.8 Mya, if he were to consume enough energy to feed a growing brain, with a reduced masticatory apparatus. In a series of experiments on snakes and rats (Boback et al., 2007; Carmody et al., 2011) Wrangham et al. showed that the energy benefits of cooking include a reduced cost of digestion and a reduced investment in immune defenses when meat is eaten (Carmody and Wrangham, 2009; Carmody et al., 2016). Schnorr et al. (2015) found that starch gelatinization by roasting, which is more relevant than cooking to early Paleolithic conditions, does not improve digestion in USO’s consumed by the Hadza but that it makes them easier to peel and chew.
The first question that was debated, and is still debated today, regarding the cooking hypothesis, is whether the archaeological record supports the habitual use of fire by humans at this early period of 1.8 Mya. The record for a fire at that early period is scant and cannot be safely assigned to human activity (Gowlett, 2016; but see Wrangham, 2017). Later, at 1-0.5 Mya, a few individual cases can be assigned to human activity (Gowlett, 2016). The cooking hypothesis predicts daily, habitual use and control of fire. This pattern is not evident in any site before 400 Kya (Gowlett, 2016), but is evident in several locations after that date (Roebroeks and Villa, 2011; Shahack-Gross et al., 2014; Shimelmitz et al., 2014; Gowlett, 2016). In summary, the status of the present archaeological evidence for fire control and use does not seem to support cooking by Homo erectus 1.8 Mya, clearly not in a habitual manner, while it does support a habitual use of fire much later than predicted by Wrangham et al., some 0.4 Mya.
Neandertals used fire, and there is even evidence that they were cooking plants (Henry et al., 2011; Hardy et al., 2012). While at some sites fire was used continuously over long periods of time (Cabanes et al., 2010), recent attention has been drawn to the fact that some Neandertal’s cave sites, where archaeological signs of fire usually are better preserved, do not show signs of fire usage during long periods of time despite extensive evidence for ongoing occupation (Dibble et al., 2017; Henry, 2017). Dibble et al. (2017) proposed that Neandertals may not have been capable of starting up fires in colder periods and were depended on natural fires, available in wormier periods. Henry (2017) proposed to consider the cost-benefit ratio of collecting wood in different circumstances as a decision rational for setting up fires. In either case, it seems that Neandertal could survive for an extended period on raw foods.
The assumption of Wrangham et al. (1999) that the physical softness of cooked food enabled the masticatory apparatus reduction in H. erectus was contested recently by Zink and Lieberman (2016), who proved experimentally that merely cutting lean meat and USOs with stone tools into smaller pieces can reduce the masticatory effort sufficiently to allow the observed reduction of the masticatory apparatus in H. erectus. They state that their findings negate the presumed need for cooking by H. erectus.
Zink and Lieberman (2016) did not take into account the significant contribution of fat consumption to the lessening of the masticatory effort of H. erectus. In an earlier paper (Ben-Dor et al., 2011) we showed that African prey, especially large ones, contain plenty of fat, thus eliminating the problem of presumably-low meat consumption, that was supposed to be solved by Wrangham et al. (1999) cooking hypothesis. Thus, it is more likely that fat, which adds zero chewing effort while providing as many calories as meat in a typical large African prey animal was a significant factor in reducing the load on the masticatory apparatus of H. erectus.
A critical assumption that Wrangham et al. use in the various papers that were cited above is that humans could not survive on a raw, uncooked, diet. The assumption is supported by the presumed fact that people on a raw diet suffer from low BMI and women suffer amenorrhea. This assertion, which is defined as “key evidence” (Wrangham, 2017) is based on a single study in a German population (Koebnick et al., 1999) whose data was obtained by gathering replies to a mail questionnaire. No physiological examinations or personal interviews were carried out. Participants were asked about their BMI and their health status. Self-reporting food questionnaires are notoriously biased (Briefel et al., 1997; Heerstrass et al., 1998; Subar et al., 2003; Archer et al., 2013). The authors describe the diet as mainly vegetarian where “staple foods like meat, dairy products, cereals and cereal products are avoided” and where “fruits and vegetables make up a high percentage of the food consumed.” In a puzzling contradiction to the above description, they do include a large group in the study which is defined as ‘meat eaters’. One possible explanation for this contradiction is that any respondent who reported consumption of any meat quantity at any frequency was included in the meat eaters’ group. In any event, it does not appear as if the “meat eaters” group consumed meat and fat in amounts that are close to those of Paleolithic humans. Also, no mention of intake of supplemental vitamin 12 or iron is made. Shortage of Iron and vitamin B12 is rampant among vegan and vegetarian populations (Alexander et al., 1994; Haddad et al., 1999) of which this group seems to be an extreme example. The diet also recommended fasting periods of between several days and several weeks. About half of the 572 participants fasted at least once a year. The most apparent shortcoming of the study, however, is that “about 55% of the participants changed to raw food diet because of their disease”. The most important disorders were a disease of the gut, allergies, asthma, and rheumatism.
To sum up, it seems that the study population is highly morbid at the base and not even representative of the present western population, let alone early Paleolithic populations like those of H. erectus. Furthermore, their raw diet of western foods could have been depleted of bio-available vitamins that are found in abundance in animal fat, meat, and organs (see Section 5.5.2), including ones that are found exclusively in animal meat and fat like vitamins A, D, and B12. In other words, it seems that the Koebnick et al. (1999) study does not provide sufficient evidence for a detrimental effect of a diet high in raw meat on humans.
Early in the 20th century several northern polar populations still ate most of their food raw (Davies and Hanson, 1965; Trowell and Burkitt, 1981). Meat that has become rotten was eaten raw (Heinbecker, 1928:462). Consumption of raw animal organs, as well as stomach content, is considered to be an adequate source of vitamin C for northern polar populations that relied almost exclusively on animal food (Fediuk, 2000; Fediuk et al., 2002).
It is interesting to note that the Hadza HG, who consume tubers, don’t cook them but merely burn them to allow peeling. Also, the tubers are highly resistant to digestion because of their high fiber content, resulting in low glucose accessibility (Schoeninger et al., 2001; Schnorr et al., 2015). USOs serve the Hadza only as a fall back food at times when other, more nutritious food like meat, honey, berries, and baobab, are not available (Marlowe, 2010:108). This phenomenon is common in recent HG groups, where dependence on plant foods typically entails a focus on seeds and nuts (Kuhn and Stiner, 2001), as is expected from their relatively higher ranking, taking into account their lower fiber content and toxin levels (Stahl et al., 1984).
Another argument that is raised by Wrangham (2017) is that H. erectus will have had to chew for eight hours a day if he was to consume raw food. Wrangham base that conclusion on a paper that describe feeding times among primates which are feeding mostly on high fiber diet (Fonseca-Azevedo and Herculano-Houzel, 2012). However, if feeding times of meat by carnivores are considered, then the feeding time is no longer an issue. For example, wolves can consume over 1 kg of raw meat per minute (Wilmers and Stahler, 2002). Inuit who consumed meat exclusively, were able to consume four kgs meat per day on a regular basis, most of it in a single meal, and ingestion of daily quantities of 15 kgs of meat per person has also been observed (Krogh and Krogh, 1914; Sinclair, 1953). Fat, which is hypothesized here to have provided up to some 50% of the calories, is the densest food and requires little mastication in relation to its caloric content.
In summary, the availability of stone cutting tools and a considerable quantity of fatty meat seems to provide an acceptable explanation for the reduced mastication already in H. erectus while the cooking hypothesis is still dependent on weakly supported assumptions, for example, that meat and fat based raw food diet would have been deleterious to H. erectus and that fire was habitually used 1.8 Mya.
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I am a Ph.D. in archaeology, affiliated with the department of Archaeology of Tel Aviv University. I research the connection between human evolution and nutrition throughout human prehistory.
